1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
The recent years have seen advancement in high-density image formation through optical scanning in image forming apparatuses, such as digital copiers, laser printers, and the like. Also, there is a requirement for a smaller beam spot on the photosensitive drum and, from the point of view of overall cost reduction of the optical scanning device, use of a resin lens.
On the other hand, any variation in environmental temperature brings about a variation in the curvature, thickness, and refractive index of a resin lens. A variation in the wavelength also causes a variation in the refractive index due to dispersion of a resin lens. This tendency of the lens characteristics to vary with temperature fluctuations or wavelength variation results in a large beam spot and causes the image focus location (point of focus) of the beam spot to vary, degrading the quality of the image.
This problem is not limited to resin lenses. Lens characteristics of even glass lenses vary with temperature and wavelength.
An improved resin lens structure is disclosed in Japanese Patent Laid-Open Publication No. 2002-214556. Japanese Patent Laid-Open Publication No. 2004-126192, Japanese Patent Laid-Open Publication No. 2003-337295, and Japanese Patent Laid-Open Publication No. H11-223783 disclose a lens with a diffractive surface. Japanese Patent Laid-Open Publication No. 2004-280056 discloses a method of forming a full color image by combining the toner images of different colors.
Resin lenses are preferable over glass lenses because they are light weight and non-expensive. Further, special figures, such as an aspherical surface, can be easily formed on a resin lens. Optical characteristics of a resin lens can be improved by forming special surfaces on the resin lenses. Because the optical characteristics of a resin lens can be improved, the same function can be performed with less number of lenses.
In other words, using resin lens can make the laser scanning device more compact, lighter, and cheaper. However, on the flipside, resin lenses are susceptible to variations in environmental conditions, and particularly, change shape and have varying refractive indices with change in the temperature. Consequently, their optical characteristics, and particularly, the power, deviate from the design value, thus causing a variation in the “beam spot diameter” which is the laser spot diameter on the surface being scanned.
The method of canceling out the variation in the power of the resin lens due to temperature variation by using a positive lens and a negative lens in the optical scanning device is very well known.
Semiconductor laser devices are employed as a light source in the optical scanning devices. These Semiconductor laser devices generally tend to output light having a longer wavelength when there is a rise in the temperature (“variation in the wavelength due to temperature variation”). The wavelength also varies due to “mode hopping”. The variation in the wavelength gives rise to chroma aberration of the optical system used in the optical scanning device, which in turn results in a variation in the beam spot diameter.
Thus, in the optical scanning device using resin lenses in the optical system and a semiconductor laser device as a light source, it has to be taken into consideration while carrying out optical designing that optical characteristics can vary not only due to temperature variation but also due to the variation in the wavelength of the light source.
Optical scanning devices (laser scanning devices) designed taking into account the variation in the optical characteristics and the wavelength of the light source due to temperature have been disclosed in Japanese Patent Laid-Open Publication No. 2002-287062, Japanese Patent No. 3397683, and Japanese Patent Laid-Open Publication No. 2000-171741 in which a power diffractive surface is provided to stabilize the optical characteristics.
In the optical scanning device disclosed in Japanese Patent Laid-Open Publication No. 2001-287062, a light source optical system lets a light beam emitted from a light source travel parallel in a main scanning direction and converges the light beam emitted from the light source near the deflective surface of an optical deflector in a sub-scanning direction. The light source optical system is a single resin optical element having more than one reflective surface without an axis of rotation symmetry, and two transparent surfaces, with a power diffractive surface provided in the transparent surfaces. In a comparative example of the disclosed optical scanning device, one power diffractive surface each is provided in a resin collimator lens that collimates the light beam emitted from the semiconductor laser device and a resin cylindrical lens that converges the collimated light beam in the sub-scanning direction. “Power diffractive surface” refers to a diffractive surface having a lens power due to diffraction.
In Japanese Patent No. 3397683, a method is disclosed for reducing and compensating for the variation in “the focal position of the beam scanning the surface” by positioning a diffractive surface more towards the light source than the optical deflector and another diffractive surface in the scanning optical system that converges the deflected beam towards the scanning surface, thereby changing the diffraction angle according to the variation in the wavelength.
In Japanese Patent Laid-Open Publication No. 2000-171741, a method of correction by “varying the focal position of the light beam scanning the surface” according to the variation in the wavelength of the light emitted from the semiconductor laser device, by making the surface of the collimator lens, which converts the light beam emitted from the semiconductor laser device into parallel rays, as the power diffractive surface.
In the light source optical system disclosed in Japanese Patent Laid-Open Publication No. 2002-287062, a single optical element must include both the transparent surface and the reflective surface, and a curved reflective surface. This is particularly easy to design for manufacturing purposes.
A collimator lens that lets the light beam emitted from the semiconductor laser device to travel parallel generally the most powerful lens among the optical elements used in an optical scanning device. Therefore, when employing a collimator lens as a power diffractive surface, such as in the instance disclosed as a comparative example in Patent document 2, or in Japanese Patent Laid-Open Publication No. 2000-171741, there is a risk of an adverse effect in the form of “degradation of wavefront aberration of the collimated light beam”. Since degradation of wavefront aberration includes the effect of increasing the beam spot diameter, using collimating lens poses a problem for high resolution images for which a very small beam spot diameter is a prerequisite.
Further, in optical scanning devices, it is common to include an anamorphic optical element as one of the optical elements disposed between the light source and the scanning surface. The anamorphic optical element has a different imaging action in the main scanning direction and the sub-scanning direction. Thus, optical scanning devices generally have different imaging actions in the main scanning direction and the sub-scanning direction. In the method of forming power diffractive surface on the collimating lens disclosed in Japanese Patent Laid-Open Publication No. 2002-287062 and Japanese Patent Laid-Open Publication No. 2000-171741, the power diffractive surface is rotationally symmetrical with respect to the optical axis. Consequently, the effect of variation in the emission wavelength of the semiconductor laser device on imaging in the main scanning direction and the sub-scanning direction cannot be adjusted independently.
In the method disclosed in Japanese Patent Laid-Open Publication NO. 3397683, the scanning optical element on which the power diffractive surface and which converges the deflected beam towards the scanning surface is generally a lens that is long in the main-scanning direction. The process of forming a diffractive surface on an operative area of the long lens is a time-consuming one as a large area needs to be covered, the low production rate of the lens pushing up the cost.